Solar electricity Grid-connected photovoltaic systems

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					Solar electricity: grid-connected photovoltaic systems                        http://www.solarserver.de/wissen/photovoltaik_grid_connected.html




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                                                                                                       Basic Knowledge
                  Solar electricity: Grid-connected photovoltaic systems
          Photovoltaic power plants convert sunlight to electric energy. The energy output of              Technical: Solar
          such PV plants will therefore reach it's peak at midday, meeting the daily energy                Collectors
          consumption peak, when the spot prices on energy are highest.                                    Technical: Solar Heating
                                                                                                           Technical: Solar Heating
                                                         Despite this economic benefit                     Systems
                                                         photovoltaic power has not yet reached
                                                         grid parity, the point at which the costs         Technical: Photovoltaics
                                                         are equal to grid power (except some              Grid-Connected
                                                         sunny islands like Hawaii that use diesel         Photovoltaic Systems
                                                         fuel to produce electricity).                     Stand-Alone
                                                                                                           Photovoltaic Systems
                                                         A PV system with a nominal capacity of 8,96
                                                         kilowatt peak, as pictured, covers the
                                                         electricity needs of two four person              Basic Knowledge
                                                         households. Picture: Sharp Electronics            Research Institutes
                                                         (Europe) GmbH                                     Solar Media

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          Net metering (US and Canada) and feed-in tariff systems                                                          search

          Since no local energy storage facilities are needed, the limiting factors sizing
          grid-connected photovoltaic systems are the available space - often a roof , the
          investment costs and the regulatory frameworks including subsidy and promotion
          programs. Such programs can include investment subsidies, net metering or feed-in
          tariffs. With net metering the inflow of electrical energy is charged up against the
          electrical consumption at the same estate, using mostly a bi-directional working
          electricity meter. This system is widely in use in the US and Canada. Since there is
          in most cases no compensation for an inflow exceeding the yearly consumption,
          photovoltaic facilities will be mostly sized to provide no more energy than consumed
          at the same estate during the year; the grid is used only as a storage facility. Within
          a feed-in tariff system on the other side, providing (like in Germany) fixed and
          guaranteed payments per kWh, more output means more profit; so facilities will be
          bigger sized.


                                                           Solar Power: Sunlight becomes the
                                                           source of electricity.

                                                           A grid coupled PV system essentially
                                                           consists of the PV panels (modules),
                                                           one or several solar inverters, a
                                                           protections device for automatic
                                                           shutdown in case of a grid breakdown
                                                           and a counter for the fed in solar
                                                           electricity


                                                           Schematic diagram of a photovoltaic
                                                           system. Illustration: LGABW




1 von 3                                                                                                                       26.09.2007 09:53
Solar electricity: grid-connected photovoltaic systems                            http://www.solarserver.de/wissen/photovoltaik_grid_connected.html


          The components of a grid-connected PV system include the PV modules, a power inverter, a
          safety device to power down at failures in the grid and an electricity meter. The                 Ads by Google
          "mains-commutated" inverter converts the direct current (DC) provided by the modules to
          alternating current (AC), simultaneously synchronizing the AC output to the AC in the grid.
          The power-generating capacity of a photovoltaic system is denoted in kilowatt peak
          (measured at standard test conditions and a solar irradiation of 1000 W per m²). Today's PV
          modules will cover an area between 7 and 10 m2 per kWp. Assumed that the modules are
          oriented to south and inclined at an angle between 30° and 35° such a PV system will
          generate in Middle and West Europe - depending on the exact latitude and other factors -
          between 800 and 1.000 kWh electrical energy per year and per kWp of nominal capacity. To
          exemplify: On a roof in Cambridge or Oxford (UK), a 4-kWp-plant with optimized module
          orientation and module inclination angle will provide about 3.380 kWh per year, at Sevilla
          (Spain) 5.640 kWh per year. The plant at Sevilla will therefore need an inverter with an higher
          input voltage than the one at Oxford.

          Some tips for planning a grid-connected PV system
                                                                                                             Ätzen mit Herz

          - Size of the PV generator                                                                         Geätzte Produkte als
          The economically optimal size of a grid-connected PV system depends mostly on                           Einzelteil oder
          different financial incentives and legal parameters, since grid parity - meaning the
          costs of photovoltaic generated electricity are equal to or cheaper than the price of             Endlosband. Auch als
          grid power - is achieved only in a very few regions today.Net metering concepts, as                        Laserteil
          they are widely in use in the US and Canada, provide - like with stand-alone
          systems - no incentive to build systems that generate more electrical energy than
          consumed at the same estate during the year; the grid replaces only a local battery                   www.aetztechnik-herz.de
          storage. Feed-in tariff systems on the other side render big systems with net excess
          profitable.




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          A PV system may cover the whole roof; the pictured solar roof (233 square meters) has a
          nominal power output of 24,2 kilowatt (kWp). Picture: Hieronimi regenerative Energien
          GmbH


          - Required module space:
          Within bigger systems mostly crystalline silicon modules are used today. To install a
          nominal capacity of 1 kWp (Kilowatt Peak) with such modules an area between
          about 7 m² (using monocrystalline cells) and 10 m² (using polycrystalline cells) is
          required.Otherwise unused pitched roofs are in many cases the most cost-efficient
          places to install a PV system, especially if they are oriented to south and inclined to
          a degree of about 30° to 37° .


          - PV Orientation and Output
          The efficiency of the photovoltaic process is at its highest if the sun rays hit the
          panel vertically. Therefore PV modules should be oriented to south (speaking of the
          northern hemisphere) and somewhat inclined; the optimal inclination angle depends
          on the location (including latitude, altitude and other factors). As a rule of thumb the




2 von 3                                                                                                                               26.09.2007 09:53
Solar electricity: grid-connected photovoltaic systems                            http://www.solarserver.de/wissen/photovoltaik_grid_connected.html


          inclination angle would be best between 3/4 and 4/5 of the latitude – resulting in
          angles of 32° to 38° in Middle and Western Europe or 30° to 36° in most of the US.
          However: Small divergences from the optimal orientation and inclination result only
          in even smaller reductions of energy output per year.

          In order to most effectively use Solar Radiation, a PV Module or Collector of a
          photovoltaic system and Solar Heating System, respectively, is aligned to absorb or
          collect as much of the radiation as possible. The radiation's angle of incidence, the
          tilt angle of the module or collector, and the azimuth angle all play roles in achieving
          the greatest possible power production.

                                                                     The azimuth angle ((ß) in the
                                                                     picture at right) specifies how
                                                                     many degrees the surface of
                                                                     the module or collector diverges
                                                                     from the exact south-facing
                                                                     direction. The tilt angle (a)
                                                                     specifies the divergence from
                                                                     the horizontal.

          Experiments show that photovoltaic systems operate most effectively with an
          azimuth angle of about 0° and a tilt angle of about 30° . Of course small variances
          in these values are not at all problematic: with the system oriented towards the
          south-east or south-west, about 95 % of the highest possible amount of light can
          still be absorbed. Large systems with arrays are fitted with electric motors which
          track the sun in order to optimise output.


          - Power inverter:
          PV systems provide direct current (DC) voltage. To feed to the grid, this DC voltage
          has to be inverted to the grid alternating current (AC) voltage by a
          »mains-commutated« or grid-tied inverter, synchronizing automatically its AC output
          to the exact AC voltage and frequency of the grid.

                                                          This MPP fluctuates during operation in
                                                          an interval depending on the radiation,
                                                          the cell temperature and the cell type
                                                          und has so to be tracked by the inverter
                                                          controlling unit.

                                                          Installation of power inverters of a 123 kWp
                                                          PV system in Germany.



                                                          Picture: IPDAD

          The second important job of the solar power inverter is to control the PV system to run near
          its Maximum Power Point (MPP), the operating point where the combined values of the
          current and voltage of the solar modules result in a maximum power output. This MPP
          fluctuates during operation in an interval depending on the radiation, the cell temperature and
          the cell type und has so to be tracked by the inverter controlling unit.




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3 von 3                                                                                                                           26.09.2007 09:53